As mentioned, Agares and Ronove would both be good for this, and each could help differently.
Agares could help more with the “academic” side of language learning. Things like memorizing vocab, learning grammar rules and usage, and improving general comprehension. It’s been a while since I’ve used this power, but I would suspect that it could help with writing and speaking fluency as well.
I have not evoked Ronove for language-learning specifically, but I have worked with the demon to improve my charisma and charm, with good results. I was only working in my native language, but I don’t see why this power couldn’t help you become more charismatic and charming in another tongue.